This invention relates to a two-way optical communication device which is capable of two-way transmission and reception of an optical signal, more specifically, the two-way optical communication device for home communication, communication between electronic devices, a LAN (Local Area Network), etc., that uses a multimode optical fiber such as a plastic optical fiber as a transmitting medium, and this invention further concerns a two-way optical communication apparatus which uses the two-way communication device.
Conventionally, regarding an optical communication device which transmits and receives signal light by using an optical fiber serving as a transmitting medium, a number of methods have been proposed for coupling an optical fiber to a light-emitting element and a light-receiving element. For example, Japanese Laid-Open Patent Publication No.222211/1987 (Tokukaisho 62-222211, published in September, 1987) and Japanese Laid-Open Patent Publication No.108511/1989 (Tokukaihei 1-108511, published in April, 1989) disclose a method for coupling a light-emitting element to an optical fiber by using a concave mirror. Referring to FIG. 9, the following explanation describes this method.
Transmitted light from a light-emitting element 105 is radiated onto a concave mirror 103. The concave mirror 103 is formed with a rotating elliptic surface. The end faces of the light-emitting element 105 and the optical fiber 102 are respectively disposed on focal positions of the concave mirror 103. Thus, the transmitted light, that is emitted onto the concave mirror 103, is reflected thereon. And the light is gathered on the end face of the optical fiber 102 and is coupled to the optical fiber 102.
However, the methods disclosed in Japanese Laid-Open Patent Publication No.222211/1987 (Tokukaisho 62-222211) and Japanese Laid-Open Patent Publication No.108511/1989 (Tokukaihei 1-108511) relate to a light-emitting module only for coupling light transmitted from a light-emitting element to an optical fiber. In the case of two-way transmission and reception, coupling optical systems are required for both transmitted light and received light, so that two optical fibers are necessary.
In order to avoid a complicated arrangement of two optical fibers, it is desirable to transmit and receive light by using a single optical fiber. However, in the case of the two-way optical communication device using a single optical fiber, it is necessary to adopt a method for separating transmitted light and received light upon coupling light to the optical fiber.
For instance, methods have been conventionally proposed including a method using a hologram (Japanese Laid-Open Patent Publication No.243905/1991 (Tokukaihei 3-243905, published in October, 1991)) and a method using a half mirror (Japanese Laid-Open Patent Publication No.115732/1998 (Tokukaihei 10-115732, published in May, 1998)).
However, these methods have a disadvantage of causing about 3 dB loss upon separating transmitted light and received light. Further, a separating element is necessary for separating transmitted and received light, so that the cost is increased and a smaller model cannot be readily achieved.
Moreover, the following method is disclosed in Japanese Laid-Open Patent Publication No.251119/1997 (Tokukaihei 9-251119, published on Sep. 22, 1997): both of a light-receiving element and a light-emitting element are disposed around an optical axis of an optical fiber and the elements are coupled to the optical fiber through a lens. Referring to FIG. 10, the following explanation describes this method.
A light-emitting element 205 is disposed on a line extended from an optical path of an optical fiber 202. A light-receiving element 204 is disposed such that a side is positioned close to a base line, which connects the optical fiber 202 and the light-emitting element 205. Received light that is propagated through the optical fiber 202 is refracted in a lens 210 and is coupled to the light-receiving element 204. Meanwhile, transmitted light that is emitted from the light-emitting element 205 is refracted in the lens 210 and is coupled to the optical fiber 202. The transmitted light and the received light are not separated from each other in this method, so that it is possible to obtain a small two-way optical communication device at low cost.
However, in such a method, the optical axes of the light-emitting element 205 and the light-receiving element 204 are brought close to each other, so that transmitted light is partly interrupted or about half of light received from the optical fiber 202 is not coupled to the light-receiving element 204, resulting in a large loss.
Furthermore, when an optical fiber having a large diameter, for example, a plastic optical fiber (hereinafter, referred to as POF) is used as the optical fiber 202, the lens 210 cannot sufficiently gather light. Particularly, in the case of high speed communication, it is necessary to reduce a light-receiving area of the light-receiving element 204 in view of electrostatic capacitance. However, in this case, it is difficult to gather multimode light, that is received from the POF having a core diameter of about 1 mm, in the lens 210 in order to obtain a high coupling efficiency on the light-receiving element 204 having a small light-receiving area.
The present invention is devised to solve the above-mentioned problem. The objective is to provide an inexpensive and small two-way optical communication device, that can transmit and receive light by using a single optical fiber with a small loss and can efficiently couple light to an optical fiber such as a POF having a large diameter without necessity for a separating element for transmission and reception, and to provide a two-way communication apparatus using the device.
The two-way optical communication device of the present invention includes a light-emitting element for emitting transmitted light to be propagated via an optical fiber, a light-receiving element for receiving light propagated via the optical fiber, and a light-gathering mirror which reflects the transmitted light so as to couple the light to an end face of the optical fiber and reflects the received light emitted from the end face so as to emit the light to the light-receiving element.
According to this arrangement, the light-gathering mirror is used at the time that light received from the optical fiber is gathered and is coupled to the light-receiving element, so that a curvature of the light-gathering mirror is varied so as to readily change a light-gathering state. For example, even in the case of an optical fiber such as a POF having a large diameter, light can be coupled to the light-receiving element with high efficiency.
Further, in this arrangement, transmitted light is coupled to the optical fiber via the same light-gathering mirror, so that transmission and reception is possible with a single optical fiber and it is not necessary to separate transmitted light and received light; consequently, a small two-way optical communication device can be obtained at low cost.
The two-way optical communication apparatus of the present invention, which includes a plurality of the two-way optical communication devices, each being optically coupled to each end of the optical fiber so as to conduct two-way optical communication, wherein at least one of a plurality of the two-way optical communication devices includes a light-gathering mirror, and the light-gathering mirror reflects light transmitted from the light-emitting element so as to couple the light to an end of the optical fiber and the mirror reflects light received from the end of the optical fiber so as to emit the light onto the light-receiving element.
According to this arrangement, the light-gathering mirror is used at the time that light received from the optical fiber is gathered and is coupled to the light-receiving element, so that a curvature of the light-gathering mirror is varied so as to readily change a light-gathering state. For example, even in the case of an optical fiber such as a POF having a large diameter, it is possible to couple light to the light-receiving element with high efficiency.
Moreover, transmitted light is coupled to the optical fiber via the same light-gathering mirror, so that transmission and reception are possible with a single optical fiber and it is not necessary to separate transmitted light and received light; consequently, a small two-way optical communication apparatus can be obtained at low cost.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.